1. Field of the Invention
The present invention relates to a dielectric thin film-forming composition that is capable of reducing dielectric loss of a dielectric thin film while demonstrating high tunability in a thin film capacitor and the like and maintaining excellent leak current characteristics, a method of forming a dielectric thin film, and a dielectric thin film formed by the method. In the present description, “tunable” refers to the ability to change electrostatic capacitance in response to a change in applied voltage, while “tunability” refers to the variability or rate of change of electrostatic capacitance.
2. Description of Related Art
Thin film capacitors, which are composed of an upper electrode, a lower electrode and a dielectric layer formed between both electrodes, are incorporated as variable capacitance elements (tunable elements) in high-frequency tunable devices such as high-frequency filters, high-frequency antennas and phase shifters. Thin film capacitors function as capacitors that cause a change in electrostatic capacitance in response to a change in voltage applied between both electrodes. A dielectric thin film formed using a perovskite-type oxide such as strontium titanate (SrTiO3), barium strontium titanate (BST) or barium titanate (BaTiO3) having a high dielectric constant is used in the dielectric layer that composes these thin film capacitors. Examples of methods used to form dielectric thin films include physical vapor phase growth methods such as vacuum deposition, sputtering or laser ablation, chemical vapor phase growth methods such as chemical vapor deposition (CVD) and chemical solution deposition (CSD) methods such as a sol-gel method or metalorganic decomposition (MOD) (see, for example, Japanese Unexamined Patent Application, First Publication No. S60-236404 and Japanese Unexamined Patent Application, First Publication No. H11-260667). In these methods, a precursor solution is prepared consisting mainly of metal alkoxide or metal carboxylate and the like, and the prepared precursor solution is deposited on a substrate by dip coating or spin coating and the like, following by going through a drying step by heat treatment and a step for decomposing and removing organic components to obtain a thin film by crystallizing to a target substance.
Thin film capacitors incorporated in high-frequency tunable devices are required to have electrostatic capacitance variability (tunability) with respect to an applied voltage, and it is preferable that the range over which electrostatic capacitance can be controlled when a voltage is applied be as large as possible, or in other words, that the thin film capacitor have high tunability. The reason for this is that as tunability becomes higher, wider resonance frequency bands can be handled with a smaller change in voltage. More specifically, if electrostatic capacitance prior to the application of voltage is defined as C0v and electrostatic capacitance after applying a voltage oft V is defined as CtV, then tunability can be represented by (C0V−CtV)/C0V×100%. For example, as shown in FIG. 1, when a voltage of 5 V is applied, although electrostatic capacitance changes from C0V when voltage is not applied to C5V, at this time, the larger the decrease in electrostatic capacitance from C0V to C5V, the higher the tunability, and the thin film capacitor can be said to have high tunability.
A tunable capacitor has been disclosed as a technology for enhancing this tunability that is capable of ensuring high tunability using a material having a high dielectric constant while maintaining desired impedance when using in a high-frequency band (see, for example, Japanese Unexamined Patent Application, First Publication No. 2008-53563).
In addition, a thin film capacitor and the like incorporated in such a high-frequency tunable device is required to not only have high tunability, but also have basic characteristics such as low leak current density and high dielectric constant. The addition of elemental Mn to a dielectric thin film composing a dielectric layer of a thin film capacitor and the like is disclosed as being an effective method for reducing leak current density (see, for example, Japanese Unexamined Patent Application, First Publication No. H2-197108 and Japanese Patent (Granted) Publication No. 4420232). In the technology disclosed in Japanese Patent (Granted) Publication No. 4420232, in addition to adding elemental Mn to a raw material liquid that forms a dielectric thin film, by baking the raw material liquid coated onto a substrate in an inert gas atmosphere containing oxygen, capacitance density can be improved simultaneous to reducing leak current density.